Steam reforming is a well known method to generate mixtures of hydrogen and carbon monoxide from light hydrocarbon feeds, which can be used, in turn, for methanol synthesis or Fischer-Tropsch process, or further for hydrogen production. Due to the strong endothermicity, the steam reforming reaction needs to be carried out at a high reaction temperature (>750° C.), and is usually performed by supplying heat to a mixture of steam and a hydrocarbon feed in contact with a suitable catalyst, typically nickel based. The catalyst is usually contained in tubes, which are placed inside a furnace that is heated by combustion of fuel, thus supplying the reforming reaction heat.
Owing to the high energy intensive character of the steam reforming process, the hydrogen production industry has been driven to improve its thermal efficiency. It is known that the thermal efficiency is improved when the overall steam to carbon molar ratio is reduced. However one of the technical barriers for decreasing this ratio to values lower than 2.0 is that carbon (or coke or soot) would form on the reforming catalyst, especially in a top-fired reformer. Carbon formation must be prevented for two main reasons: (i) its deposition on the active sites of the catalyst leads to deactivation; (ii) the carbon deposits growth can cause total blockage of the reformer tubes, resulting in the development of hot spots and overheating of the tubes.
In spite of the widespread character of the steam reforming technology it still shows several disadvantages, in particular related to its high energy intensive character and the substantial amount of greenhouse gases (GHG) emitted, such as CO2, during the burning of a part of methane feedstock in order to sustain the global endothermic balance of the steam reforming reaction.
One approach to overcome, at least partially, the mentioned drawbacks is to use autothermal reforming instead of steam reforming. Autothermal reforming (ATR) is a combination of partial oxidation and steam reforming wherein the exothermic heat of the partial oxidation supplies the necessary heat for the endothermic steam reforming reaction. This is typically carried out in a conventional, two-zone ATR reactor. In the first zone, which is typically provided with a burner, the feed is partially oxidized with oxygen or air, wherein most, if not all, of the molecular oxygen is consumed. The heat release from the oxidation reaction can raise the gas temperature as high as 1200-1300° C. The oxidation reaction is often carried out in an open volume, free of any catalyst. The hot gases then flow through a bed of reforming catalyst where the endothermic reaction between the unconverted hydrocarbons and water results in a mixture containing CO and H2 at a desired ratio.
Use of autothermal reforming is known, for example, from U.S. Pat. No. 6,375,916 B2. This publication is directed to soot free autothermal reforming of hydrocarbon feed containing higher hydrocarbons. The system comprises a pre-reformer upstream of an autothermal reformer. The autothermal reforming is carried out in a burner combustion zone and, subsequently, in a catalytic zone having a fixed bed of steam reforming catalyst. In the burner combustion zone, where thermal reactions occur, the possibility of coke formation cannot be excluded.
WO 00/78443 A1 discloses a method for generating a pure hydrogen stream for use in fuel cells. The hydrogen generation zone is provided with: (i) a pre-reforming zone; (ii) a partial oxidation zone; (iii) a reforming zone; and (iv) a water gas shift zone. The partial oxidation and the steam reforming zones are physically separated but remain in thermal contact. Also, air is fed to the partial oxidation reactor to ensure that the overall plant operating temperature is not higher than 700° C.
WO 2006/097440 A1 discloses a process for hydrogen production comprising the steps of: (i) pre-reforming of a hydrocarbon feedstock; (ii) heating of the mixture to a temperature higher than 650° C.; (iii) non catalytic partial oxidation by contacting with a source of oxygen. The non catalytic partial oxidation is performed in order to reduce the consumption of oxygen.
US 2006/0057060 A1 discloses a method for producing synthesis gas from hydrocarbons comprising: (i) a pre-reforming step; (ii) an oxidative reforming step in a catalytic ceramic membrane reactor equipped with a membrane for oxygen separation from air; (iii) steps of preheating of various process streams. The membrane based separation of oxygen from air allows to use low pressure air as an oxygen source instead of pressurised high purity oxygen which is associated with high costs.
US 2001/0051662 discloses a pre-reformer used in order to reduce the content of higher hydrocarbons in natural gas. The resulting mixture is then converted to a synthesis gas with a 2:1 H2:CO ratio and without significant soot formation in a conventional autothermal reformer. The synthesis gas is further used to produce higher hydrocarbons in a Fischer-Tropsch process.
Despite all that is known, the main drawbacks of the existing technology are (i) a higher reaction temperature to be employed to obtain a sustainable hydrocarbon conversion while affecting the selectivity to hydrogen and carbon monoxide; (ii) the presence of a burner combustion zone where carbon formation may occur.
Therefore it is desired to provide a process for the catalytic conversion of hydrocarbon feedstock with a high degree of feed flexibility and decreased soot formation, which at the same time provides an increase of the thermal efficiency of the overall process and hydrogen production and also a reduction in plant size due to the absence of burners for the catalytic conversion reaction.